Process for treating a metal or alloy by means of an electrolyte

ABSTRACT

A PATINA, PASSIVATED LAYER OR PROTECTIVE SKIN IS RAPIDLY FORMED ON A METAL WHICH IS CAPABLE OF FORMING SUCH A LAYER UPON EXPOSURE TO ATMOSPHERIC CONDITIONS OVER A LONG PERIOD OF TIME BY ALTERNATE ACTIONS OF AN ELECTROLYTE FOLLOWED BY DESICCATION AND DEHYDRATION IN A GASEOUS ATMOSPHERE WHEREIN THE ELECTRODE POTENTIAL OF THE TREATED METAL IS PERIODICALLY MEASURED, THESE STEPS BEING CONTINUED UNTIL THE ELECTRODE POTENTIAL OF THE METAL COMPARED TO A SATURATED CALOMEL ELECTRODE IS STABILIZED AT A VALUE WHICH IS HIGHER THAN 0 MV.SCE IN THE PRESENCE OF WATER.

BY ME Original Filed Jan. l5, 1969 June 18, 1974 M. PQURBAIX .3,317.795

PROCESS FOR TR ING' ETAL OR ALLOY i ANS AN E CTROLYTE I l 4 Sheets-Sheet 1 'L6 l l June 18, 1974 M. PoURBAlx 3,817,795

PROCESS FOR TREATING A METAL OR ALLOY BY MEANS 0F AN ELECTROLYTE Original Filed Jan. l5, 1969 4 Sheets-Sheet 8 FIG- 2.

E F|G 4o. FIGA.

June 18, 1974 M. PouRBAlx PRO/Ess FOR TREATING A METAL 0R ALLOY BY MEANS 0F AN ELEGTROLYTE Original Filed Jan, l5, 1989 4 Sheets-Sheet 5 June 18, 1974 Original Filed Jan. l5. 1969 FIG 7C1.

25 IIIIIHII HIHIHHII F|G 7c.

' M. PouRBAlx PROCESS FOR TREATING A METAL OR ALLOY BY MEANS 0F AN ELECTROLYTE 4 Sheets-Sheet 4- Fl G- 7b.

Field.

United States Patent O 3,817,795 PROCESS FOR TREATING A METAL OR ALLOY BY MEANS OF AN ELECTROLYTE Marcel Pourbaix, Brussels, Belgium, assignor to Centre Belge dEtude de la Corrosion, Ixelles, Belgium Continuation of abandoned application Ser. No. 791,333, Jan. 15, 1969. This application Jan. 12, 1972, Ser. No. 217,231 Claims priority, application Luxembourg, `lan. 16, 1968, 55,296/68 Int. Cl. C23f 7 `04 U.S. Cl. 14S-6.14 R 10 Claims ABSTRACT OF THE DISCLOSURE A patina, passivated layer or protective skin is rapidly formed on a metal which is capable of forming such a layer upon exposure to atmospheric conditions over a long period of time by alternate actions of an electrolyte followed by desiccation and dehydration in a gaseous atmosphere wherein the electrode potential of the treated metal is periodically measured, these steps being continued until the electrode potential of the metal compared to a saturated calomel electrode is stabilized at a value which is higher than musee in the presence of water.

This application is a continuation of application Ser. No. 791,333, led Jan. l5, 1969, now abandoned.

The present invention relates to a process for treating a metal or alloy, in particular steel, by means of an electrolyte, to form a protective skin or patina on it. It is clear that the process will not be applicable at random to any electrolyte or any metal or alloy. However the description will provide a method for easily detecting the metal-electrolyte combinations which can be used, and for rapidly finding out, by means of simple experiments, the solutions which are best suited for each case.

It is known that the surfaces of corrodible metal components, for example of non-stainless steel, can be protected against corrosion by painting or by electrolytical deposition of a less corrodible metal. These methods are sometimes very laborious and their effectiveness is often not permanent; it is therefore important to try to protect the surface by the formation of self-protective products of corrosion, which do not need any maintenance.

At present there are produced architectural Works exposed to atmospheric actions and which includes low alloy steel parts, which are not covered with paint or other exogenous protective coatings. These steels contain in particular considerable quantities of phosphorous and copper, and generally contain nickel and chromium. It is found that, in the course of years, when they are exposed to a non-marine atmosphere, they are gradually covered with protective skin of oxides. The rate of corrosion, which is high at the beginning, gradually diminishes and becomes negligible after a period which, generally about five years, varies according to the nature of the :atmosphere (rural, urban, industrial) and according to the period of the year during which its first exposure took place. During these first years of exposure, the drip water is heavily loaded with rust and because of this is brown in colour; this coloration gradually diminishes and becomes very slight after the period of about tive years in which the corrosion has become negligible, and disappears almost completely when this corrosion has become practically nil. In addition the colour of the rust formed on the steel evolves in the course of time and depends on the atmospheric conditions; initially of the usual rust colour, it remains generally brown in a rural atmosphere and becomes greyish-brown in an urban or industrial atmosphere.

ice

The implementing of these steels leads at present to certain diliiculties which restrains their employment. Firstly during the long period necessary for the formation of the skin, rain water contains corrosion products of steel, which deposit trails of rust on the stone, concrete and other materials over which this water trickles. These trails, often indelible, lead to obvious aesthetical disadvantages. Secondly, the coloration of the rust, as well as that of the skin which results from it, evolves in a manner which cannot be controlled at present, and this coloration is not always favourably accepted. Thirdly, through there not being suicient knowledge of the scientie bases which condition the formation of the skin (as Iwell as the evolution of the colour), the capacity of a steel to be covered with a protective skin cannot be rapidly assessed. An assessment of this abiilty to form a skin can only be acquired at the actual time following empirical tests of great duration, in Which different types of steel are exposed to exterior iniluences, and in which the results are observed after several years. This method does not make it possible to proceed with a rapid and really scientiiic study of the problem,

lIt is well known that when metals and alloys are subjected to alternate contacts with an electrolyte and with a gaseous atmosphere, during exposure to atmospheric actions, or to the action of fog, or to the action of tidal sea water, which Aactions cause successive moistening and drying, the rapid assessment of the corrosion presents great dithculties. The usual accelerated corrosion check methods will not give really satisfactory results; these results often cannot be reproduced; they do not correspond satisfactorily with the results of industrial practice; they do not make it possible to quickly assess the inuence of the multiple factors which condition good or bad behaviour; the influence of the nature of the metal or alloy and of its initial surface state; the inuence of the corrosive solution, whose concentration, composition and nature evolve to a large extent according to the degree of moistening or desiccation of the surface of the metal; the inuence of the gaseous atmosphere, which according to its conditions of chemical composition and according to circumstances varying with time, of temperature, irradiation and desiccation of the met-allie surface, exerts different actions on the protective nature of the corrosion products formed during the moistening periods.

There is not known at this time 1an accelerated method of imitating the mechanism of corrosion existing on steels having the ability to form a' skin. The skin which is formed on these steels is at present considered to be of a type such as nature only can produce and such as man cannot produce (see the Minutes of the Steel Congress, 1965, E.C.S.C., Luxembourg, Oct. 26-29, 1965, Progress in Steel Processing, paper by J. Dinkeloo, The new face architectural steel, p. 293, 2nd paragraph).

It is known that with a View to remedying the iirst diiiiculty, the deposit of the rust trails, it is possible to subject steels recognised as capable of forming a skin, to a certain precorrosion treatment with a certain type of electrolyte. This treatment is described in'French Pat. No. 1,479,768. The layer of rust thus formed is however still not stable in respect of water and requires a special stabilisation treatment owing to the applications of films impermeable to water, but permeable to water vapour. Besides the fact that this method is delicate, it does no make it possible to deduce from it a basis of suicient knowledge of the phenomenon of this precorrosion, in order to nd a suitable solution for each case in general, for example a metalelectrolyte combination to obtain a protective skin and a certain skin colour, nor does it enable the provision of an effective means of checking the state of advancement of the corrosion and of the eiiectiveness of the protectionr The invention has the object of providing a process for pretreating metals and alloys which forms a stable layer when they are exposed to atmospheric action.

The invention also has the object of providing a process for the rapid formation of skin which can be stabilised by simple exposure to the atmosphere.

The invention further has the object of providing a process for the rapid assessment of the corrosion, in which the knowledge of the mechanism of the formation of the skin makes it possible to provide the materials and products to be used in each case.

The invention likewise has the object of providing a process for the rapid formation of a skin to influence the quality of colour of the final skin.

The invention also has the object of providing a process making it possible to impart the ability to form a skin to metals and alloys which normally cannot.

A process in accordance with the invention is characterised in that the metal or alloy is subjected to alternate actions, on the one hand of moistening by an electrolyte and on the other hand of desiccation and dehydration, by using an appropriate electrolyte and by causing a stabilisation at high values of the electrode potential of the metal or alloy in the presence of water.

According to an embodiment of the invention, the treatment is stopped before the end of the formation of a protective skin, the completion of the treatment being eifected later by water in alternate conditions of moistening and desiccation.

As will be seen from the description, the invention likewise provides a process for rapidly assessing the corrosion of metals or alloys subjected to alternate actions of moistening and desiccation, characterised by the fact that the electrode potential of the metal alloy is measured during the periods of moistening.

This process for assessing the corrosion, which provides a rapid means for carrying out a process for the formation of a skin in a given case (for example: choice of the electrolyte to be used with a given metal, or when a definite colour is imposed), can be used likewise in other cases (for example to assess the performances of a coating of paint, subjected to atmospheric action).

The invention likewise relates to a device for effecting and checking the treatment process together with a device for checking the state of advancement of the corrosion of a metal component treated according to this process.

Specific embodiments of the invention will now be described by Way of example with reference to the accompanying drawings in which:

FIG. la represents a function of the pH value of the aqueous solutions and of the electrode potential of iron, the general theoretical circumstances of corrosion, immunity and passivity of this metal;

FIG. 1b represents, in addition to these theoretical circumstances, the potentials of insulated iron, in the absence and in the presence of oxygen, and shows in particular the potential circumstances in which the iron can be rendered perfectly passive in the presence of this gas;

FIG. 2 represents, for the special case of iron in contact with a solution capable of rendering this metal passive, the polarisation curves which express the relationship between the electrode potential and the rate of three electrochemical reactions a, b and c which can be carried out on the iron;

FIG. 3 represents a corrosion crate of iron developing under a mushroom of rust, in the presence of aerated water;

FIGS. 4a to `4d represent four types of graphs according to which it is possible to evaluate the potential of a metal or alloy subjected to alternate actions of moistening with an electrolyte and desiccation; the study of these graphs makes it possible to determine the progressive formation of layers of corrosion products and to assess from them the possible protective nature;

FIG. 5 represents a rotary device for testing and checking metals and alloys in accordance with the invention;

FIG. l6 represents a modied embodiment of the device represented in FIG. 5; and

FIGS. 7a to 7d represent removable plug devices, for checking metals and alloys according to the invention.

It has been found that when a metal or alloy is subjected to the alternate actions of moistening with an electrolyte and desiccation, the manner in which the electrode potential of this metal or alloy evolves during successive contacts with the electrolyte is intimately connected to the mechanism for the formation of the corrosion products on the metal, and to the protective nature of these products. It must be remembered that the electrode potential of a metal in an electrolyte is the difference in electrical potential between this metal and a given reference electrode, on which is produced the state of equilibrium of a reversible electrochemical reaction; the electrolyte in which the metal is immersed and the electrolyte which contains the reference electrode should be joined by an electrical bridge, for example according to the Haber and Luggin method (see for example M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Structures, publ. Pergamon Press and Cebclcor, 1966). Such reference electrode would be for example a standard hydrogen electrode (she) or a saturated calomel electrode (sce).

It should be remembered that the general conditions of corrosion, immunity and passivity of iron in the presence of aqueous solutions at about 25 C., can be represented by a graph such as FIG. 1a relating to iron (Atlas mentioned above, p. 314, FIG. 6a), in which there is arranged on the x-axis the pH value of the solution and on the y-axis the electrode potential of the metal. The two zones 1 represent the theoretical circumstances of corrosion (thermodynamic stability of dissolved `forms of the metal); the zone 2 represents the theoretical circumstances of munity (thermodynamic stability of the metal); the zone 3 represents the theoretical circumstances of passivity (thermodynamic stability of oxides ofthe metal). On pp. 76-79 of the .Atlas, such graphs have been plotted for most metals.

In FIG. lb there is shown, in addition to the theoretical circumstances of corrosion, immunity and passivity already represented in FIG. 1a, three lines representing the behaviour of iron in solutions free from chloride, not containing oxygen (line 4), and containing oxygen (lines 5a and 5b), respectively. It may be seen in this figure, that in the presence of solutions containing oxygen, iron can exist in the active state (zone 5a) and in the passive state (zone 5b). In the latter case, a potential is found which, in a general manner, is higher in proportion as the passivating oxide ilm which covers the metal is more protec tive; in the case of satisfactory protection, the potential which the metal has is situated about 250 to 550 millivolts below line b, which represents the circumstances of thermodynamic equilibrium between water and gaseous oxygen at atmospheric pressure. We have observed that there is a question of a perfectly general phenomenon: it is in this zone 5b in which are situated in stable manner the electrode potentials of all the metals and alloys perfectly passivated (rendered passive) in the presence of oxygen, subject to what is said below regarding gold and platinum, the potentials of which are slightly higher than these values.

FIG. 2 represents, for the case of iron in the presence of a solution capable of rendering this metal passive, polarisation curves which express the relationship existing between the electrode potential and the reaction currents in respect of the three electrochemical reactions 2H+l2eH2 (a), O2+4H+l4e2H20 (b) and (c) which are capable of being carried out on iron (see M. Pourbalx, Lecons sur la Corrosion Electrochimique (Lessons on Electro-Chemical Corrosion), 5th fasicule, Cebelcor Technical Report RT. 91, pp. 13-18, and FIG. 72 (1966)). In the absence of oxygen, the potential of the metal is established at the point marked o (which is situated on line 4 of FIG. 1b), which corresponds to a generalised corrosion of the metal (at a rate indicated by the line a). The presence in solution of increasing quantities of oxygen (which is reduced to water at the increasing rates indicated by lines b1, to bs) causes at rst a slight relative increase of the electrode potential, with an increase in the rate of corrosion as indicated by the abscisssae of the points 1 to 4; when the dissolved oxygen content reaches the value to which the curve b5, corresponds, the potential ofthe metal reaches the potential of passivation P; the metal is then covered with a film of oxide which, if it is protective, causes a substantial elevation in the potential of the metal; if the passivating film is perfectly protective, the potential of the metal is established at the point 6, which is the potential below which the reduction curve of oxygen be is separated from the y-axis. Such a reduction potential of oxygen exists for all metals and alloys and can be determined by suitable electrochemical experiments; in the case of metals and alloys strictly incorrosive in the presence of the solution in question, this potential is substantially equal to the potential which the metal or alloy has when it is immersed, in the electrically insulated condition, in the oxygen-saturated solution; the value of this potential depends to a certain extent on the nature of the metal or alloy: it is for these metals which are incorrodible at a relatively low oxygen reduction overvoltage and which, in the presence of oxygen, are not passivated by an oxide ilm but maintain a truly metallic surface in the state of immunity (gold and metals of the platinum family, see Atlas mentioned above, pp. 76-79) that it has the highest value (50 to 150 rnv. below the potentials indicated by the line b in FIG. 1b); for steels with a skin formed, it is situated 250 to 550 mv. below this line, i.e. in the zone 5b of this figure; it is likewise in this zone that there are situated the austenitic stainless steels, as Well as those of the other incorrodible industrial alloys. In the case of pH=7, for which the potential or line b is near +800 mv. (i.e. +550 mmm), these characteristic potentials of practically incorrodible metals and alloys are therefore established between +750 and +250 mvghe, (i.e. between +500 and 0 mv.s). Whatever the nature of the oxidising agent present in the solution in question (oxygen, oxygenated water, chromate, permanganate, etc.), the upper limit value of this zone of potentials characterising a practically incorrodible metal or alloy (for example +500 mv.sce for Water of pH=7 saturated with oxygen) would be the value of the potential which as a specimen of gold or platinum immersed in the solution.

FIG. 3 represents, it may be recalled (see Proceedings of the 2nd Steel Congress of the E.C.S.C., Luxembourg, Oct. 26-29, 1965, p. 182), a diagram of a corrosion crater of iron (No. 6) formed in the presence of water 7 and in contact with air 8; an anode 9 of low potential about -0.5 voltsshe) is corroding with the liberation of hydrogen (reactions c and a in FIG. 2) under a mushroom of rust 10, while on a passivated cathode 11 of high potential (about +0.4 volts), oxygen is being reduced (reaction b in FIG. 2)`

FIG. 4 gives four examples of graphs which represent the evolution of the electrode potential E as a function of time t for four different metal-electrolyte combinations, and in which the metal is immersed in the electrolyte for a length of time which is equal to one third of the total duration of one immersion-emersion cycle.

The electrolyte could for example be water whose composition is similar to that of rain water or fog, in the presence of which it is proposed to study the behaviour of the metal: pure water in the case of a rural atmosphere, water laden with SO2 in the case of an urban atmosphere, water laden with chloride in the case of a marine atmosphere. Each of the dashes comprising these graphs represent the evolution of the electrode potential during the same immersion. Two successive washes indicate the electrode potentials observed during two successive immersions, For a steel which is lunable in any way to form a skin by nature in the presence of the water in question, one may observe an evolution of the electrode potential in accordance with FIG. 4a. In this case, the steel remains free of any protective oxide, it undergoes a generalised corrosion affecting its entire surface, and its electrode potential remains constantly in the range of corrosion which is contained in FIGS. la and 1b, i.e., in the case of a neutral solution of pH=7, about +500 to millivolts in relation to the standard hydrogen electrode she (i.e. -750 to 350 mv. in relation to the saturated calomel electrode sce). In the case of FIG. 4b, it may be seen that the potential at the beginning of the immersion gradually becomes higher (which represents the formation of a passivating oxide at the time of drying), but rapidly falls again during immersion (which reveals that the passivating action of this action is very imperfect); the metal is then covered with corrosion craters as is shown in FIG. 3 and there are included, during the immersions, nonaerated zones (or anodes) with a low potential (about 500 rnv.) in which the metal corrodes, and aerated zones (or cathodes) with a high potential (about +400 mv.) in which the oxygen is reduced on the passivated metal; the mixed potential which can be effectively measured has an intermediate value between the potential ofthe local anodes and that of the local cathodes; a reduction in this potential during immersion reveals that the anodes with low potential develop at the expense of the cathodes, i.e. the layer of oxide formed on the metal is not protective; the water in which the metal has been immersed then contains substantial quantities of iron formed Vby corrosion of metal. In FIG. 4c, the rise in potential at the beginning of immersion is greater than in FIG. 4b and the drop in potential during immersion is less. This reveals a gradual improvement in the protective ability of the layer formed by the corrosion products; this improvement remains how ever insufficient to ensure an effective protection of the metal. However, in the case of FIG. 4d, the electrode potential is stabilised at high values and practically no further drop in potential is produced during immersion. It should be noted that steels which are inherently capable of halving a skin formed on them behave in such a way; here there has been found therefore an analogy with the formation of a skin by nature.

The mixed potential measured is then practically no longer influenced by the existence of low potential anodes, and it remains constantly equal to that of the cathodes with high potential, which, as indicated diagrammatically in FIG. 1b, in the presence of aerated solutions, are situated at about 550 to 250 rnv. below the line b (which represents, it may be recalled, the circumstances of thermodynamic equilibrium between water and oxygen at atmospheric pressure); this corresponds, for solutions of a pH near 7, to an electrode potential near to +250 to +550 mv. in relation to the standard hydrogen electrode, i.e. 0 to +300 mv. in relation to the saturated calomel electrode. It is in fact here that this can be effectively verified: it is at such values that the potential of a steel with a skin perfectly formed naturally is stabilised.

These experiments indicate that the treatment in several immersion-emersion cycles according to the invention give a protective layer similar to that formed naturally during exposure to atmospheric actions. In this latter case, the cycles are longer, and sometimes, because of prolonged moistenings or irregularity of these periods, the dehydration and recrystallisation after drying cannot be produced. A treatment is therefore provided which in the case considered in FIG. 4d, simulates in a few weeks, days, hours or minutes (cycles of one minute to two hours each) the natural formation mechanism, which takes years. The measurement of the electrode potential during and at the end of the treatment of any metal or alloy, makes it possible to predict rapidly whether a skin can be formed on this metal or not. It is also possible to observe the colour of the skin obtained, and therefore test relatively quickly which metal could be suitable for a given problem. One could then quickly study the inuence of different preliminary treatments of the metals to be subjected to the skin-forming action of water. The drop in potential during immersion, which is generally in the order of 50 to 150 mv. in 10 minutes at the beginning of the treatment of a steel without a skin formed on it, becomes lower than 2 mv. in 10 minutes for a steel with a satisfactory skin formed on it; and the iron content of the water after contact 4with steel, which exceeds l mg./litre at the beginning, finally becomes lower than 0.01 mg./litre. In the experimental conditions of these tests, the rate of corrosion of steel, which is higher than 4 millimicrons mam. per hour at the beginning (i.e. 35 microns gm. per year), finally becomes less than 0.04 mam. per hour (i.e. 0.3 am. per year). It should be noted that according to the present data in industrial practice, the rate of corrosion which skin-forma-ble steels sand-blasted in a non-marine atmosphere have, is in the order of 12 gm. per year during the rst year, 1.5 nm. per year during the second year, 1.1 am. per year during the third and fourth years, 0.6 am. per year during the iifth to eighth years, 0.2 pim. per year during the ninth to twelfth years. In certain cases of artiticial skin formation, in particular for skins produced with the use of an oxidising agent other than oxygen (chromate), there has been observed, on the one hand a transitional period during which the potential would have, during immersion, not a drop but a rise, and on the other hand a final period with stable conditions having, during immersion, drops in potential between two substantially constant high values (+150 and +120 mmm).

It should be noted that steels having the following compositions (percent) are at present considered inherently skin-forming: C 0.12 max.; Mn 0.20 to 1.00; S 0.05 max.; Si 0.20 to 0.90; P `0.07 to 0.15; Cu 0.25 to 0.55; O* 0.30 to 1.25; Ni 1.00 max.

The process used with natural water to imitate nature in its skin forming mechanism can also be used with another electrolyte, so as to form a skin in less time than with natural water (in a few hours), or so as to form a preliminary skin, which nature could achieve with the formation at least of trails of rust, or to form skins which nature does not form on skin-forming steels, and therefore of different colour and quality or further to form a skin or preliminary skin on steels which are not inherently skin-formable.

The treatment which has just been described for iron and steel can be applied to all metals and alloys which can be rendered passive with the means described above, up to the point at which the values of the potential which We have defined above are obtained, which only depend on the reduction of the oxygen on the uncorroded metal or alloy. The metals and alloys capable of being rendered passive and also the treatments to be applied to them for this purpose may be found, either from theoretical considerations (see the Atlas) or by means of suitable experiments made using the apparatus in accordance with the invention.

When it is desired to promote the formation on a metal of a skin protecting this metal against atmospheric corrosion, alternate treatments are effected of moistening, desiccating and dehydrating with an electrolyte other than natural water, but always taking care that the metal subjected to this treatment has a characteristic curve in accordance with FIG. 4d, when it would be subjected subsequently to this treatment in natural water, that is to say taking care that the metal remains or becomes capable of being given a skin. For each oxidizing solution used there exists can electrode potential value below which the reduction of the oxidizing agent could be effected; this electrode potential value, which may be considered as a measure of the oxidizing power of the solution, is the maximum Value to which this oxidizing solution can bring the potential of the steel or other metal or alloy in the case of perfect skin-formation.

Among the oxidizing solutions which may be employed may be mentioned oxygenated water and the nitrites. A particularly high degree of effectiveness could be obtained by using oxidizing agents whose products of reduction would exist in solid form under the conditions of the pH value of the solution and of the electrode potential of the metal or alloy in question. In depositing themselves on the weak points of the surface of the metal or alloy, these solid bodies can improve the protective power of the iilm of oxide producing the passivity. A first class of such oxidizing substances consists of salts which are reducible with the direct formation of a metallic or metalloidal element nobler than the metal to be rendered passive and which, on their being brought into contact with the metal, will form on the anodic areas of the metal, a deposit of this nobler metal or metalloid. Such are, as regards steel, solutions of salts of platinum and the platinoids (iridium, osmium, palladium, rhodium, ruthenium) and salts of antimony, silver, arsenic, bismuth, cadmium, cobalt, copper, tin, nickel, gold, lead, selenium and tellurium (see M. Pourbaix, Leons sur 1a Corrosion Electrochimique (Lessons on Electrochemical Corrosion), 3rd fascicule, Rapport Technique (Technical Report), Cebelcor Rt. 49, FIG. 27 (1957)). It would be possible, by proceeding in this Way, not only to improve the quality of a patina or skin, but also to modify the colour; for instance, treatment with a solution of silver nitrate will result in a black patina. The deposit of these metals and metalloids could be promoted by a galvanoplasticizing action, during which the metal or alloy is used as a negative electrode.

A second class of such oxidizing substance consists of soluble salts the reduction of which results in the direct formation of an oxide or hydroxide which is not very soluble under the conditions of the pH value of the solution and potential of the metal in question (see Atlas dEquilibres Electrochirniques (Atlas of Electrochemical Equilibria), pp. 74 and 75); this latter, being formed against the metal and/or within the layer of oxide of the metal already existing at the surface, would result in the formation of a composite layer consisting of oxides of different metals, particularly dense and adherent. Such are the solutions of permanganate, bichromate and chromate, molybdate, tungstate, nranate, vanadate, hyperosmiate and pertechnetate; these latter will give rise to films comprising a part of the Fe203 formed by the oxidization of the iron, and furthermore oxides of manganese, of chromium, molybdenum, tungsten, uranium, vanadium, osmium and technetium formed by the reduction of the oxidizing agent. It might be useful to promote the formation of solid products of reduction originating from oxidizing substances of the two classes mentioned above, by impregnating the layer of oxides before, during or after the treatment by an oxidizing agent, of substances exerting a reducing action on the said oxidizing agent; such are for instance formal, hypophosphite, hydrosulphite, hydrazine, sulphide. This permits the formation within the actual layer of oxide of insoluble solid deposits which exert a consolidating or clogging effect on this layer. For instance, the impregnation of a layer of rust by means of a solution of salt of nickel and hypophosphite makes it possible to incorporate metallic nickel in the skin. The colour of the oxides and sulphides thus formed within the actual layer could be modified by the transformation of these substances into other insoluble salts; for instance a patina or .skin containing copper sulphide and cupric oxide could be modified by the action of a bicarbonic solution, giving rise to the formation of green malachite. The protective action of the different oxides and hydroxides thus formed might be increased by the simultaneous application of subsequent application of solutions of salts giving rise to the formation of insoluble iron salts, such as the orthophosphates. Being deposited at the weak points of the film of oxide, these insoluble salts will in effect act as a brake on the corrosion reaction; such an addition of orthophosphate would be particularly useful in the case of skins called upon to resist the action of water containing chlorides.

A cleaning (degreasing, descaling or pickling) treatment, already known for steels which are to be exposed to corrosion by natural causes, will precede if necessary the skin-forming treatment. This cleaning could be effected by any known method-mechanical (by Sandblasting or shotblasting), chemical, by the addition of acid solutions (sulphuric acid, hydrochloric acid, phosphoric acid, with or Without inhibitors of cleaning-pickling), by alkaline solutions (concentrated caustic soda); electrolytic (in acid or alkaline solution). The following are recommended: cleaning-pickling operations which, using reducing substances which are not too acid (for instance sodium disulphite, hypophosphite, hydrazine) or complex-forming (for instance acetic acid, benzoic acid, citric acid, formic acid, lactic acid, malic acid, oxalic acid, picric acid, pyrogallic acid, stearic acid, tannic acid, tartaric acid or other organic oxyhydride compounds, or cyanide) produce a complete elimination of calamine and air oxide present on the steel to be treated. A particularly high degree of effectiveness can be obtained by chemical or electrolytic pickling immediately before the skin-forming treatment without any drying of the metal intervening between these two operations.

A conjoint action of cleaning and preliminary skinforming (that is to say formation of a patina or skin not protective per se but capable of becoming protective if given a subsequent treatment of moistening and drying) could be effected by the use of steel as electrolytic anode in a hot (60 C. for instance) concentrated (200 grammes per litre) solution of caustic soda, possibly with an oxidizing agent added (potassium permanganate).

A layer of oxides formed by a single tempering followed by desiccation does not give sufficient protection and does not result in the stabilization of the electrode potential at high values. In fact, after such a treatment the potential remains at low values (for instance -400 mmm), very much lower than the high values (generally speaking, zero -to +300 mv.ecs) which are necessary for obtaining satisfactory passivity. To obtain la protective layer which is in fact completely stable, a number of successive operations of moistening and drying, and also dehydration, is essential. In fact, in the case of iron and steel, the rust formed in 'aqueous solution, principally consisting of magnetite and ferric hydroxide Fe(OH)3, can only be rendered protective by the combined actions of oxidization and dehydration resulting in transformation into aFeOOH or aFe2O3H2O or into yFe2O3, and that in such a way that these substances form on the metal an adhesive and non-porous layer. This can only be obtained by the repeated actions of corrosion, oxidation and drying, accompanied by a partial or complete dehydration, one special effect of which is to render the iron oxides more insoluble. For each possible combination of metal and electrolyte there is a requisite time of desiccation and dehydration which may be determined experimentally by observation of curves of development of the electrode potential of different samples subjected to the process, but with different desiccation and dehydration times. It is likewise possible, for a metal treated in accordance with the process, to check experimentally the future behaviour of the said metal when it is subjected to the action of a given atmosphere. For this purpose the treated metal is subjected to a fresh treatment with water of the same composition as the waters of condensation and other water with which it may come into contact when exposed to a given atmosphere. It is thus possible to define for instance the treatment to be given to a steel to be exposed to a given urban atmosphere, or the treatment to be given to a steel where the actual colour of the patina is mandatory.

Heating the layer of oxide during the desiccation periods reduced the duration of these periods, accelerates dehydration and may sometimes even influence the crystalline state, and leads to a less porous layer which is more insoluble and more protective, which imparts to the metal comparatively high and stable electrode potentials, which are characteristic of satisfactory protection. This heating may be effected by irradiation.

The moistening treatment may be effected for instance by immersion, by submersion, by the application of a wash or washes, by spraying, by the action of a mist, or by means of a sponge saturated in the solution. The moistening and desiccation may be elfected hot or cold. The desiccation may be carried out in some other atmosphere than air, for instance oxygen. Generally speaking, it is a good idea to increase the oxidizing power of the moistening solutions by the injection of air or oxygen.

It is possible to increase the power of penetration of the solutions used for the moistening by adding to these wetting substances, for instance alcohols or sodium dodecyl benzenesulphonate.

It is not always necessary to treat the metal to the point of perfection, as natural causes can complete the formation of the patina or skin without too greatly colouring the drip Water.

It is clear that the metal components obtained, which are particularly intended for use in civil engineering (in the form of for example sheets, girders or assembly components) could likewise be used for other structures subjected to atmospheric action (e.g, casings). It is possible to check if a steel structure forming part of a building in the course of construction or actually finished has a suiiciently high and stable electrode potential by applying on the spot the process proposed over a small portion of the surface of the said structure, the moistening being effected by means of a plug and Wad impregnated with water from rain on the spot and provided with a reference electrode (FIG. 7), it being understood that care must be taken to see that the water in contact with the structure remains permanently aerated, for instance by keeping the contacts between the structure and the wad very light and very short; the development of the potential during the moistening will therefore be determined during the sucessive short, light applications of the wad on the moist metal. If the potential measured is found to be sufficiently high and stable it may be considered that the skin covering the steel is of good quality. If not, it is possible to apply the skin-forming process in situ, carrying out successive moistenings by any suitable method (for instance, by application of a wash or washes, by spraying, or by means of a sponge), and possibly accelerating the drying and desiccation by heating. In particular it might be useful to apply such a treatment to any places Where the layer of oxide might have collapsed during the building operations. Such treatments in situ would likewise be useful for changing the colour of any skin which did not give satisfaction.

The process could be carried out in a bath where the metal could be successively immersed and withdrawn. However, it is posible for instance to x a number of steel joists in the bath and operate by submersion, introducing an electrolyte into the bath and subsequently removing same; it is also possible to cause the electrolyte to circulate by pumping and to dry the steel structures by blowing in hot air. It is advantageous to provide the baths with a reference electrode, for controlling the treatment by measuring the electrode potential.

For carrying out experiments to determine if a metal can be made to form a skin, or to try a new combination of metal and electrolyte, or for applying a suitable technique, or again for predicting the future behaviour of a skin exposed to exterior conditions, the apparatus may be used which is represented in FIG. 5, which could 1 1 also be used in other cases where the satisfactory behaviour of the metal results from the formation on this metal of a passivity-impartng layer which will here be considered as being a skin or patina, for instance for evaluating the performance of a coating of paint subjected to the action of the atmosphere.

This apparatus comprises a container 12 filled with an electrolyte. Two samples of a metal or alloy 13 are fixed by a rod to a spindle 14 which rotates slowly (for instance, one revolution per hour). The container is filled with a reference electrode 21, such as a saturated calomel electrode. The spindle 14 is provided with two thin electric commutation plates 15. These plates are insulated in respect of one another, but each connected to the corresponding sample. The spindle 14 is likewise provided with' a commutator brush 16, which is connected to one of the voltage measurement terminals 17. 'Ihe other terminal is connected to the reference electrode. The commutator brush is give such a position that the electrode potential of the immersed element is measured at the terminals 17. An incandescent lamp serves to heat the sample when withdrawn. Thanks to the rotation of the spindle the samples are immersed and withdrawn successively and the commutator, controlled by the rotation of the same spindle, makes it possible to measure the electrode potential with the same voltage measuring instrument. There may be fixed to a given spindle a multiplicity of samples insulated electrically in respect of one another and each connected to a corresponding Iamella or thin plate and forming part of a commutator on the spindle. The commutator brush or brushes are fixed in such a Way that it is possible to measure for each sample at least the voltage value at the beginning and at the end of each immersion. There may also be associated with each sample its own millivoltmeter without making use of electrical commutation to measure the\electrode potential of a multiplicity of samples with the same millivoltmeter.

The results given by the voltage measuring instrument could be fed to a curve tracing instrument with several tracks. The choice of track will be determined by the position of a commutator mounted on the spindle. 'Ihe values registered could equally well be fed to a converter of analogical values into numerical values, the output side of which would be connected to an ordinator or other instrument for treatment or recording numerical values.

The electrolyte could be made to circulate slowly, by allowing it to fall drop by drop from the tube 18. An overflow container 19, connected by means of a siphon 20 to the principal container serves for the evacuation. The level of the container 19 determines the level of the electrolyte, and hence the proportion between the times of immersion and emergence.

In a variant of the apparatus described above, the measurement of the electrode potentials is effected with reference to a reference electrode 21 rendered solid with the metal sample, and whose junction Siphon 22 with the electrolyte adjacent to the metal is applied against the said metal (FIG. 6). This arrangement makes it possible to measure the electrode potentials not only during the immersion periods but also after these periods, and that till complete desiccation of the surface of the metal has been attained.

`Other variants are likewise possible, all comprising a device for measuring the electrode potential during the moistening periods, and utilizing other mechanisms to perform the operations of moistening and drying. Successive operations, of pumping to submerge the sample, and draining, may likewise be effected.

If it is desired for instance to check on the spot components already installed and fixed in position, the process may be effected by means of a wad or plug impregnated with the electrolyte used (FIGS. 7a and 7b). This plug 23 is fixed in a fitting 24 in which is fitted a reference electrode 25.

It is advantageous to provide the apparatus, as indicated in FIG. 7c, with a device making possible an aecurate adjustment of the position of the plug on the moist surface, in such a way as to avoid any crushing of the film of water covering the metal. This device could be for instance a micrometric device 26 allowing the material forming the plug 23 (for instance wadding or asbestos fibres) to come into contact with a drop of water 27 situated on the metal without touching the metal itself.

It is also advisable, particularly when using a reference electrode with chlorinated electrolyte (calomel or silver chloride electrode) to take precautions to prevent this electrolyte from diffusing till it reaches the water in contact with the metal to be examined. This could be done for instance, as shown in FIG. 7d, by inserting a protective plug 28 between the electrode 2S and the working plug 23, and/or by the use of a removable electrode 25.

It is advisable when using the plug appliances to take the precautions indicated above, that is to say using light applications, possibly successive, permitting a permanent aeration of the moist component.

The following examples illustrate the invention.

EXAMPLE 1 A phosphor-copper type steel, supposed to be capable of having a skin formed on it, where the skin-forming process normally takes about 5 years, was subjected, immediately after having undergone a cleaning (pickling) treatment with sulphuric acid, to alternate treatments of immersion in a solution of oxygenated water (0.3 grammes H2O2 per litre) circulating with a rate of flow of 140 cc. per hour, and emergence into air under infrared radiation, at the rate of 30 minutes immersion and 90 minutes emergence. The electrode potential of the steel at the beginning of immersion, which was 500 millivolts in respect of the ecs staturated calomel electrode at the beginning of the treatment, reached |190 millivolts after 2l days treatment. 'Ihe drop in potential occurring during a given period of immersion, which was millivolts in l0 minutes at the beginning of the treatment, was reduced to 5 millivolts after 21 days of the treatment. The iron content of the solution in circulation, which was 0.6 parts per million at the beginning of treatment, Iwas reduced to 0.02 parts per million after 21 days, which substantially corresponds to the behaviour of a steel within a patina or skin formed on it of normal quality. The skinforming time has thus been reduced from about 5 years to about 21 days.

EXAMPLE 2 A phosphor-copper steel type, supposed to be capable of having a skin formed on it, was subjected to alternate treatments in a solution of silver nitrate (0.17 grammes AgNO3 per litre) circulating with a rate of :flow of 140 cc. per hour, and emergence into air under infrared radiation, at the rate of 15 minutes immersion and 45 minutes emergence. The electrode potential became stable after 10 days at +220 millivolts ecs, with a drop of 5 millivolts in 10 minutes in the course of immersion. The iron content of the solution in circulation was established at 0.02 parts per million. The colour of the patina or skin, which was brown in the absence of silver nitrate, became black in the presence of this substance.

EXAMPLE 3 A phosphor-copper type steel, supposed to be capable of having a skin formed on it, was subjected, after having undergone a cleaning (pickling) treatment by immersion in a sodium bisulphite solution (0.2 grammes NaHSO3 per litre), to immersion/emergence treatments in aerated distilled water at 20 C. at the same rate of speed as in Example 2, for 26 days.

During the treatment with the visulphite, the electrode potential at the beginning of immersion varied between 600 and -510 mmm, and drops in potential were found during immersion between 40 and 110 mv. in ten minutes. rIhe iron content of the solution was then 37 parts per million which, under the test conditions, corresponded ho'ur. Afterthree days treatment the potential had reached mmm and the iron content had-.been-.reduced to 0.1 parts per million, corresponding to a corrosion speed of 0.4 millimicrons-perhour-( about 0.4 --mic'o'ns per year, say). The steel can then be considered as pre-skinformed, that is to say covered-With a sk in which is not perfectly protective but can easily Ibe rendered 'perfect by subsequent treatment. A potential of +100 mv.ecs was reached after 7 days, 150 mv.m after 11 days and +200 mvm, after 23 days; the iron content of the water was then reduced to 0.01 parts per million, corresponding to a corrosion speed of 0.04 milliriiicrons per hour (say about 0.04 microns per year, which is practically zero). This makes it possible to consider the steel as perfectly resistant to corrosion.

EXAMPLE 4 Percent, max. 0.17 0.06 0.05

The electrode potential only reached 0 mvc, (pre-skinforming) after 1'4 days (instead of 3 for the skin-forming steel considered in Example 3), +100 mmm after 18 days (instead of 7), and +150 mvxec, after 24 days instead of l1). After 27 days the potential had reached +200 mmm. This slower development of 'potential for the steels considered to be incapable of having a skin formed on them than for steels o n which a skin can be formed, makes it possible to differentiate clearly between the dierent steels of these two classes. Furthermore, the obtaining of a potential of +200 mv. after 27 days treatment discloses the fact that this treatment when prolonged long enough makes it possible to form a skin on a steel supposed to be incapable of having a skin formed on it.

EXAMPLE The steel capable of having a skin formed on it considered in Example 3, after having been subjected for 10 minutes to a cleaning (pickling) treatment with a solution of NaHSO3 (100 grammes per litre), was subjected to immersion/ emergence treatments in hot aerated distilled water (50 C.) at high speed (40 seconds immersion and 80 seconds emergence). The pre-skin-forming potential (0 mv) was reached in 16 hours.

EXAMPLE 6 The steel capable of having a skin formed on it considered in Examples 3 and 5, after having been subjected for 10 seconds to a celaning (pickling) treatment with a 192 grammes per litre solution of 'citric acid, was successively rinsed with water, imml'ersed for 15 seconds in a. 160 grammes per litre solution Aof copper sulphate, rinsed with water, immersed for tenminutes in a hot (85) solution of 10 grammes per litre sodium nitrate, and subjected to immersion/emergence treatments in hot water (50) at a fast rate (40 seconds immersion and 80 seconds emergence). The pre-skin-forming potential (0 fvoltse) was reached in 5 hours.

EXAMPLE 7 A phosphor-copper type steel supposed to be capable of having a skin formed on it, was subjected for 4 days to alternate operations of immersion in aerated distilled water and emergence into the air under infrared radiation, resulting in a rusted metal having a potential of -250 ivfgc's' at'the beginning of immersion, with a drop in potential of 50 mv. in l0 minutes during immersion; the rust formed was not protective therefore. This steel was then subjected to cycles of immersion in a chromic solution (2 grammes K2Cr0., per litre) and emergence into the air under infrared radiation, which resulted, in the presence of this solution, in a potential of -225 mueca with a rise in a potential of 5 mv. in 10 minutes in the course of immersion.

Repeating on the steel thus treated by the chromic solution the treatments with distilled water effected before the chromic treatment, stabilization of the potential between and +120 mvg., was found; the rust formed was therefore protective.

I claim:

1. Method of rapidly forming a protective passive layer on a steel body capable of forming a passive layer upon exposure to atmospheric conditions over a long period of time, which comprises the steps of placing said steel body in contact with an aqeuous electrolyte solution, and removing said steel body from said aqueous electrolyte solution and subjecting the same to desiccation and dehydration in an oxygen-containing gaseous atmosphere, and alternately repeating said steps of placing the steel body in contact with an aqueous electrolyte and desiccating and dehydrating in an oxygen-containing gaseous atmosphere while periodically measuring the electrode potential of said steel body in contact with said aqueous electrolyte solution, said steps being continued until the electrode potential of said steel body as compared to a saturated calomel electrode is stabilized at a value which is higher than 0 mv.sce in the presence of water, thereby obtaining said steel body with a protective passive layer thereon in a time which is a small fraction of the time for formation of such protective passive layer under normal atmospheric conditions.

5 2. Method according to claim 1 wherein the steel has the following composition: up to 0.12% of C, between 0 .20 and 1.00% of Mn, up to 0.05% S, between 0.20 and 0.90% Si, between 0.0 7 and 0.15% P, between 0.25 and` 0.55% Cu, between 0.30 and 1.25% Cr, up to 1.00% Ni, and the balance substantially Fe.

3. Method according to claim 1 wherein said electrolyte solution is selected from the group consisting of aqueous solutions of silver nitrate and of copper sulphate.

4. Method according to claim 2 wherein alternate electrolyte solutions are used, the rst of said electrolyte solutions being selected from the group consisting of aqueous solutions of silver nitrate and of copper sulphate, and the second of said electrolyte solutions being oxygenated water.

5. Method according to claim 1 wherein said steel body capable of forming a passive layer upon exposure to atmospheric conditions over a long period of time is subjected to a pretreatment of pickeling by a solution of an agent selected from the group consisting of reducing bisulte and citric acid prior to said alternately repeated steps of contact with an aqueous electrolyte and desiccation and dehydration in an oxygen-containing gaseous atmosphere.

. 6. Method according to claim 1 wherein after determining the number and times of said desiccation and dehydration steps, said method is carried out on ther'same type of steel body utilizing said number and times of desiccation and dehydration steps without periodically measuring the electrode potential of said steel body in contact with said aqueous electrolyte solution.

7. Method according to claim 1 wherein said steel body is stabilized at a value between 0 mv. and 300 mv.

8. Method according to claim 1 wherein said aqueous electrolyte solution isan oxidizing solution.

9. Method according to claim 1 wherein said oxygencontaining gaseous atmosphere is air.

1 '16 10. Method according to egim 1 wherein said aqueous A OTHER REFERENCES electrolyte solution is aerated- L.- McAulay vet al.: Jornalbfthe Chemcal Soc,

References Cited .pp' 85-'92 January 1929' Y Y 'I UNITED STATES PATENTS 5 RALPH S. KENDALL, Primary Examinar 2,077,450 4/1937 Weisberg et al 1486.14 1 f U S 'CL X R 2,059,053 10/ 1936 Stareck 14S-6.35 14s- 624, 6 35 3,479,229 11/ 1969 Becker 1486.35 X 

